It is The Book of Name Signs, Naming in American Sign Language by Samuel J. Supalla (1992, DawnSignPress); I just happen to have it. 
Aside from the "rule" aspect, sometimes it's just safer to let a trusted Deaf friend create the name sign. There are too many horror stories about novice signers creating inappropriate (even unintentionally obscene) name signs.
But watch out for the Deaf trickster who thinks it's great sport to give an innocent hearie a nasty sign name. Yeah, it happens. 
Sometimes it is just convenient for hearing people to create name signs for people who are known to the Deaf community but are not part of the Deaf community. Sometimes just using the person's initials are enough.
I think some hearing people ask the Deaf person to do the "naming" because they're afraid to offend the Deaf community by doing the naming themselves.
Maybe they think they are bestowing some kind of honor on the Deaf person by asking him to do the naming. Almost like a spiritual ritual. If that's the case, run with it. Close your eyes, strain for inspiration, touch the person's head, mumble a chant, bestow the name sign, jump up, and shout. If you can sprinkle some fairy dust, even better. (Just kidding, folks.) 
